172 BULLETIN OF THE BUREAU OF FISHERIES. 



Host species. Parasite species. 



Polypterus congicus (fish of Lake Tanganyika) haplocepltala, adult female. 



Polypierus senegalis (fish of the White Nile) haplocephala, adult female. 



Rutilus rutilus (European roach) esocina, adult female. 



Stizosiedion canadense (sauger) cruciata, copepodid larvae. 



Food. — Whatever discussion there may be as to the food of copepod parasites 

 belonging to other famihes, it would seem as if there was no room for any question 

 among the Ivcmseidae. When a parasite breaks through the skin of its host and burrows 

 into the underlying tissues, we can only conclude that it is in search of suitable food. 

 Moreover, since the tissues themselves are not destroyed, the blood and the lymph which 

 bathes the tissues are the only possibilities left, and these make up the food of the 

 present genus. 



Economic relations. — ^The deliberate burrowing of the parasite into the flesh of the 

 host must cause the latter keen torture, and it will be noted that the list of hosts includes 

 many of our game fishes. If this genus ever became abundant it would furnish a serious 

 menace to our fresh-water fisheries. But fortunately it is held in check in many ways, 

 so that there is little danger of its accomplishing much harm. 



During the two free-swimming periods the larvae are a part of the plankton and 

 are subject to all its dangers and vicissitudes. The first of these periods includes both 

 the nauplius and metanauplius stages, while the last includes the final copepodid stage, 

 or so much of it as is occupied in the search for the final host. Thus the entire time 

 during which the larva swims about freely is much longer than in the Lemseopodidse 

 and approaches closely to that of the Caligidae. 



Again the necessity for two hosts operates against the genus ; if it is difficult to find 

 one host it is twice as difficult to find two. And the ratio is increased by the possibility 

 that the larva may be carried by the first host where it can not find the second one. 

 The more complicated the life history the more numerous are the chances of failure. 

 And the species of the present genus, together with the other Lemseans, have the most 

 complicated development amongst all the parasitic copepods. All these things operate 

 so much against them that they are the least numerous of the parasites; one may often 

 examine hundreds of fishes without finding a single specimen of the genus. And they 

 never occur in any considerable numbers upon a single fish; one specimen to each host 

 is by far the most common mode of distribution. 



EXTERNAL MORPHOLOGY. 



The morphology of the genus has already been well worked out by Brtihl, Claus, 

 Hartmann, and others, but they used only what could be seen through the body wall 

 or discovered by ordinary dissection. 



So far as known no investigator has ever employed serial sections or double staining. 

 These agents bring out many additional facts, explain others that have heretofore 

 seemed obscure, and modify considerably some of the interpretations that have been 

 given. In order that the present account may form a complete whole the substance 

 of what was pubUshed by the above-named authors is here included in a condensed 

 form and to it are added the new facts and explanations. 



General body form. — The body of the adult female is made up of three parts or 

 regions — the cephalothorax, the free thorax, and the abdomen. (See fig. 69, pi. xiv.) 



Dorsally the head is well differentiated from the first thorax segment, but ventrally 

 the two are completely fused. From the sides and sometimes from the dorsal surface 



